This invention relates to a sputtering technique for making a film, and more particularly to a sputtering target suitable to form a film having a uniform thickness on a substrate with its uneven surface, e.g. a silicon wafer in the process of making an integrated circuits.
It is found that conventional sputtering apparatus provide a film with reduced steps coverage at its outer side steps as disclosed in SEMICONDUCTOR WORLD, 1984 September pages 125-131. It should be noted that the outer side step is referred to a dead angle of the step when the step is seen from a center of a film forming object, e.g. wafer.
The reduced step coverage may increase the resistance of wiring when a film is formed on the wiring material. Thus, the resultant semiconductor devices may exhibit their deteriorated performance or the wirings may be cut so that the production yield of the integrated circuits will be reduced and the reliability of the semiconductor devices will not be assured.
An example of the conventional planar magnetron sputtering devices, which uses a wafer serving as a film forming object and a target stationarily in the face of the wafer, serving as a supply source of particles of film material through a sputtering phenomenon was proposed in U.S. Pat. No. 4,444,635 "Film Forming Method" to Kobayashi et al.
When a wafer having a diameter of 125 mm is used as a film forming object in the above sputtering device, a wafer-target distance of 70-90 mm and a diameter of 120-140 mm of the plasma ring generated on the target are taken for uniform film forming on the wafer. In such a location, however, the amount of the film formed on outer side steps is primarily smaller than that on even parts and inner side steps which are referred to as surface of the steps capable of seeing from the center of a film forming object, thereby substantially deteriorating the step coverage of the outer side steps.
In order to obviate such a disadvantage, it can be proposed to increase the amount of the film to be formed on the outer side step by generating a plasma ring which is larger in its diameter than the diameter of a wafer, and also to carry out the film forming with a shortened distance between the wafer and target. When the film forming is actually carried out in such a manner, the film thickness becomes thicker in the periphery of the wafer, i.e. uniformity cannot be obtained on the entire wafer.
To compensate for the above defect, it can be proposed to carry out the film forming in the manner of simultaneously or individually generating not only the larger plasma ring but also a smaller plasma than the wafer diameter. In the sputtering apparatus using an ordinary planar target, however, this film forming using the smaller plasma ring does not provide a fundamental solution. Namely, the amount of the film formed on the inner side step in the periphery of the wafer and the even part is only increased whereas the coverage of the outer side step is substantially the same as or decreases in the conventional sputtering apparatus.